The bioluminescence of Photobacterium phosphoreum is emitted from a novel protein (Mr 20,000) containing 6,7-dimethyl-8-ribityllumazine as a non-covalently bound, fluorescent prosthetic group. This "lumazine protein" has been purified to homogeneity. Bioluminescence arises in some protein-protein interaction with luciferase which will be studied both kinetically and by direct techniques, i.e. sedimentation measurements and decay of fluorescence emission anistoropy.